1. Field of the Invention
This invention relates to a conical tension membrane.
2. Prior Art
A structure which may be used for a similar purpose such as a garden umbrella, which affords a substantial area of shade and weather protection, has a flexible cover, centrally fixed near to the top of the central staff, and lying against an assembly of arms pivotally connected to the staff. The perimeter of the cover being secured at intervals to the outer ends of the arms. The arms may be extended by sliding up the shaft a sleeve connected to intermediate positions on all of the arms by pivoted struts, with a catch or pin to hold this sleeve when fully raised. When the garden umbrella is thus raised, its fabric cover is tensioned tangentially as the arms are raised but there is little radial tension applied to the cover. As a consequence of this, the cover is likely to flutter in high winds. A further disadvantage is the difficulty encountered in raising it, as the arms, bearing the weight of the cover, must be extended by forcing the sleeve upwards towards the struts, initially lying close to the shaft, to swing outwards, A very considerable effort is required to do this, both during the initial movement of the sleeve and during its final movement which is resisted by the tangential tensioning of the cover.
The prior art also includes the form of "umbrella" as described by Australian Patent Number 572753. In this previous device the conical form fabric is tensioned and restrained by arms pushed downward and outwards to the fabric. The fabric is usually unable to prevent significant deflection of the arms and overstressing of the top of the fabric under high downward wind pressures even when it incorporates pliable reinforcing strips.
This is not only because of the limited stiffness of such strips, but more importantly because they are attached to the fabric over its whole radial extent. Significant downward deflection of the outer end of the arms is necessary to straighten these radial seams from their pre-curved geometry, and before extension of these strips can occur. It is not until these strips can extend that significant restraining forces can be generated. This effect is particularly important in structures with few arms and high fabric curvatures.
Furthermore, this type of structure is not suited to resisting high upward wind pressures or particularly a combination of downward pressure on the windward side and upward pressure on the leeward side.
The device in APN 572753 has two design problems:
Firstly, the structure as per Australian Patent Number 572753 proved in use to not be stable in high winds due to lifting of the tips of the arms under strong negative pressure. This is resolved by the use of hold down cables. PA1 Secondly, APN 572753 use webbing reinforcement integral in the seams. This proved, in use, not to be sufficiently stiff to adequately support the arms, which is what the design requires. PA1 Thus, the design of the structure in APN 572753 had adequacies in the role that was required of it, namely, to be a stable structure, capable of resisting high winds. PA1 The invention described herein meets this basic requirement.